If you are not used to use MySQL, why do you want to mess around with it?
If you use a standard setup the groups table is called gl_groups. gl_users is tha table for your users. In those you have to find the right ids. Then add an entry in gl_group_assignments with the ight ids.
And if you really looked in your database, what did you think "gl_group_assignments" was used for?
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